1. Field of the Invention:
The present invention relates to a photographic film cassette of which rotation of a spool causes a film leader to advance to the outside of a cassette shell, more particularly to an improvement of a construction for advancing the leader.
2. Description of the Related Art:
A known photographic film cassette includes a photographic filmstrip (hereinafter referred to as film) positioned so that a film leader does not protrude from a cassette shell prior to loading the cassette in a camera, as disclosed in U.S. Pat. No. 4,887,776. Simple film-advancing mechanisms of the camera are used with the type of cassette and include a construction which rotates a spool to unwind the film, thereby causing the leader to move through a film passageway and exit from the cassette.
In this construction, a roll of the film wound about the spool is prevented from loosening in order to transmit rotation of the spool to the leader. The spool is constituted of a pair of spool pieces slidable relative to each other. A first spool piece is associated with a first flange, whereas the second spool piece is associated with a second flange. When a predetermined one of the spool pieces is rotated, a cam mechanism associated with both spool pieces operates so as to shorten the interval between the flanges by sliding the spool pieces.
The first flange is provided with a circumferential lip which projects from its periphery and has a bevelled lip inclined on its inside face for contact with a side of the outermost turn of the roll of film in order to prevent the roll from loosening. When sliding the spool pieces, the outermost turn of the roll is clamped between the second flange and the bevelled lip so that the roll is rotatable integrally with the spool and is kept from loosening. A spring washer is disposed between the first flange and a lateral inside surface of the roll chamber of the cassette shell for biasing the first flange in a direction tending to clamp the roll in the widthwise direction of the film.
There is, however, a problem in such a cassette in that the filmstrip may have an unusually small width due to shrinkage over time or a change in ambient conditions. Also, the cutting process, during manufacture of the film, may yield errors in size. In conventional cassettes, it has been impossible to firmly clamp the roll of an unusually narrow film between the flanges. This causes the likelihood of a failure in advancing the leader of such a film.
Another disadvantage of this type of cassette is that when the film is wound up into the cassette shell, its lateral edge is successively pressed and flexed by the lip. The lateral flexure of the lateral edge causes a large resistance to transportation of the film, so that a torque necessary for rotating the spool is excessively enlarged.
Because the roll becomes larger during wind-up, it is possible that flexure at one portion along the lateral edge may cause a downstream portion of film to turn around the outer periphery of the flange and thus be trapped between the flange and the cylindrical surface of the roll chamber of the cassette shell. The lateral edge, thus, would fail to be in contact with the bevelled lip. Such a difficulty is particularly likely because the spring washer constantly presses on the flange inward.